Such a display device, meanwhile known as an electrowetting display (EWD), may be used in a number of apparatuses such as a portable computer, for example a notebook or a game computer or a handheld computer, such as a personal digital assistant (PDA). Such a display may also be used in a watch or in a digital photo display. Since an electrowetting display combines many attractive properties such as high colour brightness, video speed, and low power consumption, it constitutes a substantially better alternative for the well known and nowadays widely used liquid crystal display (LCD). Because of said combination of properties the electrowetting display is very suitable for use in smaller handheld apparatuses, like mobile phones, digital picture- or video-cameras etc.
An electrowetting display is known, for example from PCT patent application WO 03071346. As described therein, this display comprises a large number of electrowetting elements, which constitute the display picture elements (pixels). Each element is provided with a first fluid and a second fluid, which fluids are immiscible. In the height- or thickness-direction of the display the fluids are confined by a first support plate, or base plate, and a second support plate. Both the first and the second support plate may be common to all electrowetting elements. At its inner side the first support plate is provided with a transparent electrode, which is covered by a (hydrophobic) layer less wettable to the second fluid. The first fluid is for instance oil and the second fluid may be water. In a first state, when no voltage is applied between the electrode and the second fluid, the first fluid adjoins the first support plate, i.e. covers the hydrophobic layer, whilst the second fluid adjoins the second support plate. When a voltage is applied, the first fluid layer moves aside or breaks up into small droplets. Since parts of the second fluid now penetrate the first fluid layer, the electrowetting element becomes partly transparent so that it can transmit light. The first fluid of an element is confined by the first support plate and, in the lateral direction, by element walls.
Several embodiments of this basic design of the electrowetting display are possible. For example the viewing side, i.e. the side facing an observer, may be the side of the second support plate so that the first support plate may constitute a base plate, and the electrode arranged on this plate may be transparent or reflective. The transparent electrode may be a layer of indium tin oxide (ITO), or a very thin layer of an electrically conductive material or a so-called open layer, i.e. a layer provided with holes. The viewing side may also be the side of the first support plate. The element walls may protrude from the first support plate and/or from the second support plate. The electrowetting display may be configured to spatially modulate, according to the image to be displayed, either a transmitting beam or a reflected beam. The electrowetting display may also include more than two supporting plates.
As described in PCT patent application WO 2004104670, the height of the element walls is substantially smaller than the width of the gap between the supporting plates. This measure provides the advantage that the brightness of the pixels will be enhanced, especially when the pixel size is reduced to increase the definition of the display. In addition it provides the advantage that the second, conducting, fluid portions of the individual elements are interconnected across the whole display. It is true that the height of the first fluid upon contraction may in fact be larger than the actual wall height. However, it has been confirmed experimentally that in the contracted shape the oil remains within the pixel.
Although the concept of the electrowetting display has proven to be very feasible, it has turned out that unexpected problems may occur. It has been established that some of the manufactured displays do not satisfy the specifications, and also that a display, which passed manufacture control may deteriorate during use so that its performance becomes unacceptable. Neither these problems nor their cause have been reported up to now.